A metamaterial is an artificial material obtained by periodically arranging unit elements called cells and having electromagnetic characteristics not existing in the natural world. The metamaterial is applied to, for example, an electromagnetic bandgap structure, an antenna, and a lens having a negative flection ratio. The electromagnetic bandgap structure is a metamaterial having a specific frequency band (to be referred to as an electromagnetic bandgap hereinafter) that suppresses the propagation of electromagnetic waves. The electromagnetic bandgap structure having this electrical characteristic is applied to a band-stop filter, to suppress the mutual interference between antennas, and the like. Since the electromagnetic bandgap structure also functions as a magnetic wall, applications using this property are being examined. For example, Japanese Patent Laid-Open No. 2009-044556 has disclosed an antenna thinning technique of closely arranging a metal plate and antenna by using the electromagnetic bandgap structure.
A mushroom structure is a metamaterial to be mounted on a printed circuit board. The mushroom structure is a structure in which cells each obtained by connecting a conductor layer (generally, a ground layer or power supply layer) and a patch conductor by a connection conductor (generally, a single via) are periodically two-dimensionally arranged (for example, Japanese Patent Laid-Open No. 2002-510886). In the mushroom structure as shown in FIG. 1 of Japanese Patent Laid-Open No. 2002-510886, a series capacitor CL is formed between adjacent patch conductors, and a parallel inductor LL is formed by the connection conductor. These elements are the elements of a left-handed system. In addition, a series inductor LR is formed by the patch conductor, and a parallel capacitor CR is formed between the patch conductor and conductor layer. These elements are the elements of a right-handed system. A frequency between a parallel resonance frequency ωsh (=1/sqrt(LL×CR)) and series resonance frequency ωse (=1/sqrt(LR×CL)) is the electromagnetic bandgap. When the size of the cell is decreased, therefore, the series capacitor CL, parallel inductor LL, series inductor LR, and parallel capacitor CR generally also decrease, and the frequency of the electromagnetic bandgap rises. Accordingly, the conventional mushroom structure requires large cells in order to achieve a low-frequency-band electromagnetic bandgap. This makes it difficult to mount the mushroom structure on a printed circuit board, particularly, a small-sized, high-density printed circuit board.
Accordingly, a metamaterial (electromagnetic bandgap structure) that lowers the frequency of the electromagnetic bandgap by increasing the parallel inductance LL by the shape of the connection conductor has been proposed. For example, each of Japanese Patent Laid-Open Nos. 2009-004779 and 2009-224567 has disclosed a metamaterial in which the conductor length is increased by forming the connection conductor by connecting a plurality of vias and a plurality of conductor lines in series, thereby increasing the inductor of the connection conductor, that is, the parallel inductor LL.
Unfortunately, even the structures as described in Japanese Patent Laid-Open Nos. 2009-004779 and 2009-224567 are insufficient to meet the recent demands for high-density, small-sized packaging.